Desilication of high-silica Indian coal fly ash by alkali leaching with KOH and NaOH: A comparative study

Abstract

ABSTRACT Indian coal fly ash generally comprises 50–60% , and 25–35% can potentially replace bauxite as an aluminum resource. Aluminum recovery from coal fly ash is highly dependent on removing amorphous silica, activating the inactive mullite phase, and increasing the ratio. This work thoroughly investigated and compared the alkali-desilication operation by two widely used alkalis, and , based on four operational parameters: liquid-to-solid ratio, or concentration, leaching time and temperature. Using SEM, EDX, and XRD, the morphology and phase of fly ash and solid byproducts were analyzed. was more effective than in dissolution at all operational parameters. A 20 wt% concentration, 5:1 liquid-to-solid ratio, 2hrs leaching time, and 100°C temperature were the optimum removal conditions for silica from fly ash using . With , the ideal desilication conditions were obtained at 28 wt% concentration, 3hrs leaching time, and 150°C temperature. The highest ratios were obtained under the same optimal desilication conditions. Using and , the ratio was enhanced from 0.58 to 1.26 and 1.45, respectively. The main zeolitic products obtained were Hydroxysodalite in -treated fly ash and Linde F zeolite and Kalsilite in -treated fly ash.